In wireless telecommunications, multi-path fading is a known cause of fluctuation in received signal levels and therefore a known cause of deterioration in communication.
Diversity reception has been widely performed, as a method of reducing this fading. For example if two antennas are separated at predetermined distances from each other, there is a significantly reduced probability that the signals from both antennas are simultaneously attenuated to the same extent, hence achieving a higher reliability. For example, signal to Interference margins can be increased by as much as 10 dB when two antennas are used rather than one. Various kinds of diversity reception methods are known.
In most cases, if "perfect" diversity is assumed, a system requires apriori knowledge of the received signal strength indication (RSSI hereafter) at all antennas which can be selected. For many prior art diversity systems, such information can only be achieved through the use of dual receivers. In such an arrangement, the RSSI of the signal received on each antenna is continuously monitored, and the best antenna is selected. This implementation adds considerable cost to a radio receiver.
Some prior art solutions teach diversity receivers which use two or more antennas and one receiver circuit. An antenna selection circuit switches between antennas in response to received signal strength indication generated by the receiver. However, without a dual receiver, the unused antenna must be sampled periodically, resulting in bit hits (lost or corrupted bits) or frame hits (lost or corrupted frames) in the received data whenever the "tested" antenna has a poor RSSI.
Furthermore, these prior art diversity systems have in the past been of particular relevance to base stations and have been of less relevance to user terminals in part due to the requirements of having the antennas physically separated by a minimum distance of half a wavelength. This physical separation of the antennas has typically made it impractical to incorporate such techniques in small, compact mobile terminals. Also, as stated, diversity techniques typically utilize dual receivers which are acceptable for base stations but are not cost effective for terminals.
However, in many ways the problem of multi-path fading presents more of a problem to a fixed wireless access terminal than to a mobile terminal because the subscriber terminal is stationary and therefore the terminal is unlikely to be moved by the user in response to poor reception as a result of a deep fade.
Furthermore, even in situations where multi-path fading does not represent a significant problem, the quality of reception can still be poor as a result of co-channel interference (CCI) in the forward link. This problem is particularly serious in high capacity cellular networks where the desire to reuse frequencies many times can degrade the signal to interference ratio (SIR). However, conventional prior art diversity solutions designed to switch antennas based on a comparison of the received signal strength indication (RSSI) between antennas will not typically solve such a problem.
Another problem not solved adequately by the prior art as known by the inventor herein, is that some users may have their radio link performance limited by the forward link signal strength while other users may have their forward link quality limited by co-channel interference (CCI) due to a high subscriber density. Furthermore, typically, there exists insufficient apriori knowledge to predict which of these two problems is the dominant cause of poor reception. Furthermore, both of the above impairments (i.e., fading and CCI) can occur at various times, and independently of each other. Therefore, there exists a need for an improved diversity system which can improve reception regardless of which impairment causes degraded reception.